The present invention relates generally to direct methanol fuel cells, and, more particularly, to air breathing fuel cells using methanol vapor fuel.
The use of methanol as a direct fuel supply for fuel cells is now the subject of numerous fuel cell development programs because the supply and storage of the liquid methanol is much simpler than a hydrogen fuel supply. A typical methanol feed fuel cell includes a membrane-electrode assembly (MEA), typically an anode, cathode, and a polymer electrolyte membrane, sandwiched between porous backings for supplying fuel and oxidant reactants to the MEA. The following electrochemical reaction occurs at the fuel cell anode:
CH3OH+H2Oxe2x86x92CO2+6H++6exe2x88x92
Simultaneous with the anode reaction, the electro-reduction of oxygen occurs at the cathode:
O2+4H++4exe2x88x92+H2O 
The overall reaction is then given by:
CH3OH+1.502xe2x86x92CO2+2H2O 
Most early direct methanol fuel cell (DMFC) research was performed using a vapor-phase methanol fuel feed. A typical vapor feed set-up used a carrier gas sparged through a conventional humidifier bottle containing a methanol-water mixture. The methanol vapor concentration was controlled by humidifier bottle temperature and/or by the composition of the methanol mixture in the bottle. In a practical system, fuel management of a methanol vapor fuel supply is not trivial because of position-tolerance and evaporation heat issues.
Subsequently, the simpler approach of supplying liquid methanol became the preferred method of introducing the methanol fuel, and is subsequently used in the majority of current direct methanol efforts. The use of the liquid feed requires that the methanol must be supplied as a fairly dilute methanol-water mixture. Dilute solutions are necessary to minimize the amount of methanol lost through the membrane to the cathode (air) side.
Substantial effort has been expended to develop membranes that are highly ionic conductive and still minimize methanol cross-over, but no entirely satisfactory membranes yet exist. Instead, in order to minimize the cross-over, relatively thick membranes are used. For example, a commonly used membrane for direct methanol cells is Nafion 117 (7-mils or about 175 xcexcm thick). This is in contrast to hydrogen fuel cells, in which membranes as thin as 1-mil (25 xcexcm) are now commonly used.
The thicker membrane increases cell resistance, but the methanol cross-over is decreased to a degree that the net performance and efficiency of the cell favors the thicker membrane. Despite the use of the thicker membrane, feed concentrations are typically less than about 1 Molar methanol to provide the best overall results. Amongst the limitations of using the dilute feed, however, are 1) a fuel management system is required to mix and recirculate the dilute feed, and 2) a large majority of the water from the cell needs to be recovered to make up the dilute feed. Another issue, whether liquid or vapor feed, is the removal of the by-product carbon dioxide per the above anode reaction equation. Unless recovered from the effluent CO2 stream, methanol vapor is released from the system. In consumer applications, venting methanol may be undesirable. Consequently, a subsystem to recover or separately oxidize the methanol is probably required in many instances.
A particular area of interest for direct methanol fuel cells is for use as small battery replacements for portable electronics and the like. Because of the relative simplicity of a dilute liquid methanol feed approach and the early indications of high performance capabilities, most battery-replacement research is ongoing with dilute liquid methanol fuel feed. In one direct methanol fuel cell configuration, unpressurized air (ambient pressure air) is the source of oxidant for the methanol fuel. Air-breathers using stored dilute methanol as the liquid feed have been pursued as portable power supplies (see, e.g., U.S. patent application Ser. No. 09/726,836, filed Nov. 30, 2000), although batteries have a higher energy content than available from the dilute liquid methanol fuel (about 2% methanol in rim water) now being considered. In this case, the CO2/methanol effluent aspect is not addressed. In another approach, neat (100%) methanol is carried as the fuel supply, but fairly involved subsystems are then required to address the fuel mixing, water recovery, and carbon dioxide release issues. This approach is then similar to that used in larger systems, but is directed to miniaturizing the subsystems and their electronic controls to provide a sufficiently compact package.
These approaches are not ideal for small battery-replacement applications, either because of excessive cost and complexity or low fuel energy density. Another important issue that may arise in such applications is positional tolerance, i.e., the ability of the fuel cell to operate irrespective of cell orientation.
The present invention is directed to a direct methanol fuel cell system that manages the water balance and the carbon dioxide release without the need for separate subsystems, that allows the use of neat methanol fuel, and that provides positional tolerance with respect to supplying the fuel.
Various objects, advantages and novel features of the invention will be set forth in part in the description which follows, and in part will become apparent to those skilled in the art upon examination of the following or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.
The present invention includes, in its broadest aspect, a fuel cell having an anode and a cathode and a polymer electrolyte membrane located between anode and cathode gas diffusion backings. A methanol vapor fuel supply is provided. A permeable polymer electrolyte membrane having a permeability effective to sustain a carbon dioxide flux equivalent to at least 10 mA/cm2 provides for removal of carbon dioxide produced at the anode by reaction of methanol with water.
Another aspect of the present invention includes a fuel cell having an anode and a cathode and a polymer electrolyte membrane located between anode and cathode gas diffusion backings and using methanol as a fuel. A superabsorpent polymer material is placed in proximity to the anode gas diffusion backing to hold liquid methanol or liquid methanol solution without wetting the anode gas diffusion backing so that methanol vapor from the liquid methanol or liquid methanol-water solution is supplied to the membrane.
Yet another aspect of the present invention includes a fuel cell system with at least one fuel cell having an anode and a cathode and a polymer electrolyte membrane sandwiched between anode and cathode gas diffusion backings and using methanol as the fuel, each fuel cell defining an anode plenum. A superabsorbent material is contained within the anode plenum for modulating methanol access to the anode and holding a liquid methanol-water solution in proximity to the anode gas diffusion backing. A fuel reservoir contains liquid methanol or methanol-water solution as a liquid fuel and a feed line connects the fuel reservoir to the anode plenum of each fuel cell.